Method and reagent for determining nitrogen oxides

ABSTRACT

A measured quantity of gas such as atmosphere containing nitrogen oxides is contacted with Hochheiser solution comprising dilute base with or without a foaming agent, however, now containing small amounts of OsO4, XeO3 or perxenate whereupon at least about 90 to 95 percent of the NO2 is absorbed yielding nitrate and nitrite in the solution. Nitrate may then be converted to nitrite as by reduction in a cadmium amalgam cell. Then the amount of nitrite is determined photometrically using Saltzman&#39;&#39;s reagent comprising n-(1-naphthyl)-ethylene diamine and sulfanilic acid which forms a dye specifically with -NO2. The collection efficiency of the Hochheiser solution is increased from 25-70 percent to above 90 percent yielding a more accurate indication of NO2 content.

United States Patent [191 Crawford et al.

[ METHOD AND REAGENT FOR DETERMINING NITROGEN OXIDES [75] Inventors: Richard W. Crawford; Lester P.

Rigdon, both of Livermore, Calif.; Richard J. Thompson, Hillsborough, NC.

[73] Assignee: The United States of America as represented by the United States Atomic Energy Commission, Washington, DC.

[22] Filed: Apr. 13, 1972 [2!] Appl. No.: 243,877

[ Dec. 4,1973

3,574,552 4/ 1971 Rakowski 252/408 X Primary Examiner-Robert M. Reese Attorney-John A. Horan [57] ABSTRACT A measured quantity of gas such as atmosphere containing nitrogen oxides is contacted with Hochheiser solution comprising dilute base with or without a foaming agent, however, now containing small amounts of 050 XeO or perxenate whereupon at least about 90 to 95 percent of the N0 is absorbed yielding nitrate and nitrite in the solution. Nitrate may then be converted to nitrite as by reduction in a cadmium amalgam cell. Then the amount of nitrite is determined photometrically using Saltzmans reagent comprising n-( l-naphthyl)-ethylene diamine and sulfanilic acid which forms a dye specifically with -NO The collection efficiency of the l-Iochheiser solution is increased from 25-70 percent to above 90 percent yielding a more accurate indication of NO content.

9 Claims, No Drawings METHOD AND REAGENT FOR DETERMINING NITROGEN OXIDES BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The determination of nitrogen oxides in effluent gases from various equipment or in the atmosphere is of interest in various industrial operations as well as for determining atmospheric pollution and products of industrial processes.

Two techniques are widely used to sample air for nitrogen oxides. For discrete sampling, the air is pumped through a fritted glass tube-that is immersed in an absorbing solution. The resulting bubbles pass through the solution, thereby providing the necessary gas/liquid interface for a chemical reaction. (c.f.

M. E. Jacobs and S. Hochheiser, Anal. Chem. 30, 426 1958).

G. B. Morgan, C. Golden, and E. C. Tabor, New and Improved Procedures for Sampling and Analysis in the National Air Sampling Network, in Automation in Analytical Chemistry (Mediad, lnc., White Plains, N. Y.,

I966, Vol. 1, pp. 526-533.

B. E. Saltzman, Anal. Chem. 26, 1949 (1954).

F. P. Scaringelli, F. Rosenberg, and K. A. Rehme, Environ. Sci.'Techn0l. Vol. 4, N0. 11, 924 (1970).

For continuous monitoring, the air is'passed directly over the flowing absorption solution in a spiral coil. (c.f.

L. Pierce, Y. Tokima, and K. Nishikawa, J. Air Pollut. Contr. Ass. 15, 204 (1965).

Hochheisers solution, which consists of 0.1-M NaOH and a foaming agent, is commonly used to collect discrete field samples. This solution converts only 35 i 5 percent of the N to nitrite ions (NOf) plus some nitrate ions (NO3 The overall conversion efficiency is 30 to 50 percent and is somewhat dependent on the pore size of the fritted bubbler and on the foaming characteristics of the solution. Nitrite can be determined alone or as the sum of nitrite plus nitrate by reducing the nitrate before analysis.

A second medium is Saltzmans solution, which is useful for both discrete sampling and continuous monitoring. The diazotizing and coupling reagents are mixed with a buffered solution so that the diazo dye forms continuously as the sample is collected. This reagent converts about 72 percent of the N0 gas to the diazo dye, but both the reagent and the dye are unstable and the samples must be analyzed soon after they are collected.

Both solutions react only with N0 any NO present in the gas phase must first be oxidized to N0 The efficiency with which Hochheiser's solution absorbs N0 is low and variable, requiring careful control of pore size and foaming characteristics to obtain reasonable reproducibility. Also, Saltzmans solution is not stable enough for field use.

A need therefore exists for a reagent which can absorb greater proportions of the nitrogen oxides, have improved stability and otherwise be adapted to provide more complete, reliable and accurate determinations of nitrogen oxides in a gaseous mixture.

SUMMARY OF THE INVENTION The invention relates, generally, to the determination of nitrogen oxides in gaseous atmospheres and, more particularly, to such a determination wherein more complete and accurate absorption of nitrogen dioxide is effected by means of a Hochheiser reagent containing small amounts of OsO XeO or perxenate.

' A general object of the invention is to provide a procedure using an improved dilute alkaline reagent for more accurately determining nitrogen dioxide content of a gaseous material. 7

Another object of the invention is to provide an improved procedure for determining nitrogen dioxide in ya gaseous atmosphere wherein the gaseous atmosphere is contacted with a Hochheiser type solution containing from about 2X10 M to about 3X10 M of OsO.,, XeO or saturated Xe'Og" (perxenate) to absorb at least about to percent of the nitrogen oxides therefrom, the absorbate is electively subjected to a reducing treatment to convert any N0 therein to nitrite, wherein there is then added n-( l-naphthyl)- ethylene diamine and sulfanilic acid to convert the nitrite into a diazo dye, and then colorimetrically determining the nitrite'content of the solution to indicate the nitrogen oxide content of the initial gaseous atmosphere.

Other objects and advantageous features of the invention will be apparent in the following description:

DESCRIPTION OF A PREFERRED EMBODIMENT A predetermined quantity of Hochheiser type absorbent reagent containing an additive agent in accordance with the invention may be contacted with a measured atmospheric sample containing nitrogen oxides, particularly N0 in a variety of known absorbing or sampling apparatus. One type of such apparatus is the sequence sampler described at pages 426-428, Volume 30, No. 3, March 1958, ofAnalytical Chemistry. See also Archives of Environmental Health, March 1962, Volume 4, pp 254-258, which describes the National Air Sampling Network Gas Sampler, a modified version of which is used by the Environmental Protection Agency (EPA). Another system including static and dynamic scrubbers is disclosed in UCR- L-5l057, a report originated by Lawrence Livermore Laboratory, Livermore, California, and authored by Lester P. Rigdon and Richard W. Crawford. Any such air sampling or gas scrubbing apparatus providing adequate contact between the solution and atmospheric sample may be used.

The Hochheiser type reagent may comprise about 0.1 M NaOH and a foaming agent, e.g., 0.2 volume percent n-butyl alcohol as in usual practice. However, the foaming agent is generally not required'with the present reagent since absorption efficiency is so markedly increased. The concentrations of NaOH may be varied over a range of at least about 0.05 to about 0.25 molar.

The Hochheiser solution is modified, in accordance with the invention, by addition of particular agents, the preferred additive comprising osmium tetroxide (OsO in amounts of at least about 2X10 M to at least about 4X10 M, where above additional amounts generally do not increase absorption efficiency. Absorption efficiencies of the order of 95 percent are consistently obtained using this agent. Other additives which are effective are XeO in similar amounts as 050 and saturated solutions of Na XeO (perxenate) which has a low solubility in the reagent.

The manner in which the additives operate is not entirely understood. They are strong oxidizing agents; however, the major proportion of the N is adsorbed to yield nitrite, a reduced form, rather than N0 It is believed that the absorbing improving action may be catalytic in nature which is rather surprising since an oxidation reaction might well be expected to occur in view of the oxidizing nature of the additive agent. About 90 to 95 percent of the N0 is absorbed to form nitrite.

The absorbate or an aliquot thereof may then be analyzed by adding a coupling agent such as n( lnaphthyl)-ethylene diamine dihydrochloride and sulfanilic acid in dilute acetic acid solution to form a diazo dye which is determined colorimetrically. The absorbate solution may also be treated with cadmium amalgam at a pH of about 8.2to 8.8 to reduce any nitrate formed during absorption to nitrite so that the total N0 may then be determined. Nitrite or nitrite plus nitrate formed during absorption may accordingly be determined. The pH of the final solution should have a value of about 1.9 to 2.3 for maximum color development.

Calibration curves for colorimetrically indicating the nitrogen oxide content of the absorbate and consequently of the atmosphere can be constructed by add ing known amounts of NaNO or NaNO standard solution to the I-lochheiser solution and/or to contact known volumes of atmospheres containing known quantities of N0 with the reagent of the invention. NO does not appear to interfere with the N0 determination.

Further details illustrating the manner in which the reagent of the invention is employed will be set forth in the following specific examples.

Apparatus of the character described in the aforesaid report UCRL-S 1057 was used. Such apparatus includes a gas calibration portion for preparing appropriate gas mixtures by intermixing regulated flows of N0 with N or air as well as a scrubbing and analysis section. The latter section included fritted glass scrubbers containing measured volumne of the reagents being compared, followed in sequence by a glass coil dyamic scrubber through which Saltzmans solution at a rate of 1.4 ml/min was directed to determine any residual N0 passing through the static scrubbers. 1.0 ml/min of the Saltzman absorbate was directed through a colorimeter connected to a recorder to indicate residual N0 content in the gas passing through the fritted glass absorbers.

The research manifold, as described in UCR- L-5l057, was used so that the rate of flow of N0 could be selected and maintained, and thus the total amount of N0 for a given time could be calculated. Also, a pure carrier gas could be used so no interfering species would come in contact with the solution. Dry, compressed tanks of commercial N and air, were used as the carrier gas. The N0 gas was obtained from the N0 permeation tube described in UCRL-5l057.

Each experiment was started by setting a flow of about 400 ml of carrier gas and a calculated amount of and analytical system reached a steady state. The analysis was then compared with, and verified the pre-set rate of flow of N0 It should be noted that the rate of flow of the carrier gas does not affect the rate of flow of N0 and that changing the rate of flow of the carrier gas changes the concentration of N0 in the gas mixture. The rate of flow of the carrier gas is not critical, and was selected to optimize the efficiency of the scrubbing coil. The stopcocks were then turned so as to force the gases through the fritt immersed in the scrubbing solution. The fritt serves to disperse the gas and provide contact with the solution. The gases were passed through the solution to the coil scrubber and were continuously analyzed. By observing the continuous analyzer one could calculate the amount of N0 passing through the fritt (static) scrubber. At the end of a run the static scrubber was switched out of the gas stream, and the rate of N0 flow again verified by the continuous analyzer. The rate of flow of N0 was assumed to have been constant if the continuous analyzer showed the same value for N0 at the beginning and end of the sampling period, and further that these values agreedwith the flow rate calculated for the flow rate settings.

The static scrubbing solutions were quantitatively transferred to a volumetric flask and diluted to a known volume with the same concentration of NaOH solution as the test solution. The solution was then analyzed for nitrite and for nitrite plus nitrate. A blank scrubbing solution of the same composition as the test solution was used to establish the zero base line for the analysis. The percent scrubbing efficiency of the test solution was then calculated by comparing the total NO flow during the test period to the total nitrite plus nitrate found in the solution.

MOre particularly, the absorbate was analyzed for nitrite utilizing proportioning pumps which mixed 1.20 ml/min of sample, 0.80 ml/min of air and 0.60 ml/min of reagent which was passed through the colorimeter to yield the concentration of nitrite therein. For determining nitrite plus nitrates, samples of the absorbate were mixed in proportions of 1.20 ml/min to 0.32 ml/min of buffer yielding a pH of 8.5 therein which was then passed through a cadmium amalgam reduction cell and then treated as in the case of the nitrite analysis.

REAGENTS All organic chemicals are Eastman Red Label reagents. Other chemicals are reagent grade and are selected for low nitrate and nitrite impurities. Deionized water is used to prepare all solutions.

STOCK COUPLING REAGENT Dissolve l g of n-( l-naphthyl)-ethylene diamine dihydrochloride in 1 liter of water. Keep tightly capped and refrigerated when not in use.

SALTZMANS SOLUTION Dissolve 5 g of sulfanilic acid in 900 ml of water, then add 50 ml of glacial acetic acid and 50 ml of the stoc coupling reagent.

REAGENT FOR NITRITE IN HOCI-IHEISERS SOLUTION Dissolve 10 g of sulfanilic acid in 870 ml of water, then add ml of the stock coupling reagent and 30 ml of concentrated H PO REAGENT FOR NITRITE PLUS NITRATE IN l-IOCHI-IEISERS SOLUTION Dissolve g of sulfanic acid in'892 ml of water, then add 100 ml of the stock coupling reagent and 8 ml of concentrated H PO BUFFER SOLUTION Prepare a solution that is 0.29 M in I-ICI and 0.10 M in H 80 Mix this solution with Hochheisers solution in the analytical manifold and check the pl-I-with a pH meter. Add I-ICl or NaOI-I as necessary to obtain a pH of 8.5.

REDUCTION COLUMN Add 100 ml of l-wt percent I-IgCl to 10 g of 30- to 60-mesh cadmium powder or filings, stirring for 3 min. Allow the powder to settle and decant the liquid. Wash the amalgam several times with water until the water becomes clear. Prepare a column that has a length of 20 cm, an inside diameter of 5 mm, and a constriction in the middle. Bend the column into a U shape so that it will not drain when it is not in use. Plug the column with glass wool at the constriction and fill one side with amalgam, taking care not to let the column become dry. Cut off the ends of the column and attach tubing to each end so that the above solutions can be pumped through the column.

STOCK NaNO STANDARD SOLUTION STOCK NaNO STANDARD SOLUTION Dissolve l.848 g of NaNO in 1 liter of water so that I it has the same molarity as the NaNO solution. To prepare working solutions containing 10 to 25 ug of N0 per milliliter (when the N0 is reduced to N0 dilute aliquots of the stock solution with water.

NITROGEN-DIOXIDE STANDARD Use a 2-cm-long, thick-walled diffusion tube, maintaining it at 28 i 0.l C as described in UCRL-S I057.

NITRIC-OXIDE STANDARD Use two standards, one containing 95 ppmof NO in N the other containing 9 ppm of NO in N EXAMPLE I The above procedure was followed, using N and air as carrier gas for several concentrations of 050., in several concentrations of NaOH. Experiments were also conducted with those solutions with 0.2 volume percent n-butyl alcohol as a foaming agent in the scrubbing solution, and without the use of n-butyl alcohol. These experiments established the following: (1) that the scrubbing efficiency was the same for a given solution when either compressed air or N was used as the carrier gas; (2) the scrubbing efficiency was the same for a given concentration of 050., when the concentration of NaOH was varied between 0.05 and 0.25 molar; (3) that 40 ml of a solution containing 0.05 to 0.25 molar NaOH and 2.5 X 10' molar OsO or more will remove 90 percent or more of N0 from a stream of N or pure air in the scrubbing system described; (4) the efficiency endures for at least 24 hours for flow rates of N0 in the range of 3-184 ng per min. in various flow rates of the carrier gas; and (5) that the addition of n-butyl alcohol does not change the scrubbing efficiency of the test solutions; (6) that XeO and Na XeO added to NaOI-I scrubbing solution improves the scrubbing efficiency by about a factor of two over the use of the same concentration of NaOH alone. Data for a n ber of expgin ents are shown in Table l.

'lnfi li l fii ifilect of "reagent adiflition on solution-scrubbing eflicleney for N02 (Hochheiser solution used to remove N01 from air).]

A second set of experiments were conducted to evaluate the efficiency of the system for removing N0 from ambient air within the laboratory. The experiments were conducted in the same manner as in (I), but no N0 standard was admitted and a pump was used to compress the ambient air and supply it to the sampling manifold. These tests shows the no N0 was passing through the static scrubbers in a 24 hour period if they contained 2.5 X 10 M OsQ, or more, by observation of the continuous analyzer. And that both nitrite and nitrate ions were present in the static scrubbers. Test solution of NaOH with no OsO added invariably passedsome N0 as shown by the continuous analyzer.

EXAMPLE m A third set of tests were conducted to evaluate the scrubbing solutions in a field gas sampling of the type used by the Environmental Protection Agency (APCO), and to get a direct comparison of it with the research manifold. The field kit utilizes a vacuum pump to pull air at a high flow rate through a glass manifold.

containing five sampling tubes in parallel. The flow of air into each tube is controlled by a calibrated critical flow orifice. In our experiments, the air entered the sampling tube through an upturned glass fritt disc, 1 cm in diameter, and having pore size between and microns. The fritt was attached to a glass tube and extended to the bottom of the tube containing the test solution. The air was exhausted through the critical orifice to the atmosphere after passing through the solution. Forty milliters of each test solution was placed in polypropylene tubes about 3 cm diameter and 16 cm long. Polypropylene caps fitted with the inlet and outlet devices were sealed to the tubes containing the test solutions to prevent air leakage. All other connections for air flow through the manifold were made of teflon or glass tubing.

EXAMPLE IV Simultaneous tests were conducted using the re search sampling manifold and the field sampling manifold. Two test solutions were used concurrently in the field sampling manifold. The research manifold static scrubber contained 40 ml of 2.5 X 10 M 050 in 0.25 M NaOl-l. One sampling tube of the field sampling manifold also contained 40 ml of 2.5 X M 050 in 0.25 M NaOH and the other one contained 40 ml of 0.25 M NaOH, but no OsO The critical flow orifices caused 220 ml of air per min. to flow through the field sampling manifold test solutions, and that flow rate was also set on the research manifold. The results for two 24 hr. sampling periods are shown in Table 2. The continuous analyzer was used to test for N0 passing through the static test solution in the research manifold, and no N0 was found to pass during a 24 hr. period, therefore, it was concluded that the test solution containing OsO removed all of the NO, from ambient air when sampled through the field sampling manifold. Table 2. Nitrogen oxide absorption efficiency of two manifolds and solutions.

N0 collected (ng) APCO manifold Research manifold Run 0.25M NaOH in 0.25M NaOH in 0.25M NaOH 1 5,200 10,100 10,100 2 5,400 9,300 9,400

While there has been described in the foregoing what may be considered to be preferred embodiments of the invention, modifications may be made therein without departing from the teachings of the invention and it is intended to cover all such as fall within the scope of the appended claims.

What we claim is:

1. In a process for colorimetrically determining the nitrogen dioxide content of a gaseous atmosphere, the steps comprising:

contacting a known volume of said atmosphere with an aqueous solution comprising about 0.05 to about 0.25 molar NaOH and an agent selected from the group consisting of 050,, XeO and Na XeO in amounts effective to promote the absorption of N0 to form nitrite therein, and adding sulfanilic acid and n-( l-naphthyl) ethylene diamine to the absorbate containing said nitrite to form a diazo dye for colorimetric determination.

2. A process as defined in claim 1 wherein said agent comprises a material selected from the group OsO, and XeO present in a concentration in the range of about 2X10- to about 4X10- molar.

3. A process as defined in claim 1 wherein said agent comprises Na XeO in a saturated solution thereof.

4. A process as defined in claim 1 wherein said absorbate is reduced by contact with cadmium amalgam at a pH of about 8.2 to 8.8.

5. A process as defined in claim 1 wherein the pH of the diazo dye final solution is in the range of about 1.9 to 2.3. I

6. A process as defined in claim 5 including the further step of colorimetrically determining the concentration of nitrite in the absorbate for comparison with standard nitrite calibration curves to indicate the N0 content of said atmosphere.

7. An improved reagent for absorbing N0 from a gaseous atmosphere to form nitrite therein suitable for colorimetric determination of the N0 in a process as defined in claim 1 comprising an aqueous solution having a basicity equivalent to about 0.05 to about 0.25 molar NaOH and including an agent selected from the group consisting of OsO XeO and Na,XeO dissolved therein in amounts effective to promote the absorption of N0 to form nitrite therein.

8. A reagent as defined in claim 7 wherein said agent comprises a material selected from the group consisting of OsO and XeO in concentrations of about 2X10 to about 4X 10 molar.

9. A reagent as defined in claim 7 wherein said agent comprises Na XeO as a saturated solution. 

2. A process as defined in claim 1 wherein said agent comprises a material selected from the group OsO4 and XeO3 present in a concentration in the range of about 2 X 10 5 to about 4 X 10 4 molar.
 3. A process as defined in claim 1 wherein said agent comprises Na4XeO6 in a saturated solution thereof.
 4. A process as defined in claim 1 wherein said absorbate is reduced by contact with cadmium amalgam at a pH of about 8.2 to 8.8.
 5. A process as defined in claim 1 wherein the pH of the diazo dye final solution is in the range of about 1.9 to 2.3.
 6. A process as defined in claim 5 including the further step of colorimetrically determining the concentration of nitrite in the absorbate for comparison with standard nitrite calibration curves to indicate the NO2 content of said atmosphere.
 7. An improved reagent for absorbing NO2 from a gaseous atmosphere to form nitrite therein suitable for colorimetric determination of the NO2 in a process as defined in claim 1 comprising an aqueous solution having a basicity equivalent to about 0.05 to about 0.25 molar NaOH and including an agent selected from the group consisting of OsO4, XeO3 and Na4XeO6 dissolved therein in amounts effective to promote the absorption of NO2 to form nitrite therein.
 8. A reagent as defined in claim 7 wherein said agent comprises a material selected from the group consisting of OsO4 and XeO3 in concentrations of about 2 X 10 5 to about 4 X 10 4 molar.
 9. A reagent as defined in claim 7 wherein said agent comprises Na4XeO6 as a saturated solution. 